very few major supermarkets do much butchering anymore, so they don't have bones lying about in the back. you can look for packages labeled "soup bones" or "neck bones". oxtail works too. if you have access to a latin/spanish or asian grocer, you will have better luck getting bony cuts.
for chicken broth i buy backs, heads and feet at an asian grocer.
lastly, google grass-fed beef farmers in your area. they will sell you bones and off-cuts for pretty cheap.
btw, i usually do up 15-18 pounds of bones at a time, so i only have to make broth a few times per year. i eat it more days than not, and that wouldn't happen if i didn't have bags and bags in the freezer.
As I ate the oysters with their strong taste of the sea and their faint metallic taste that the cold white wine washed away, leaving only the sea taste and the succulent texture, and as I drank their cold liquid from each shell and washed it down with the crisp taste of the wine, I lost the empty feeling and began to be happy and to make plans.
– Ernest Hemingway